This invention relates generally to the measurement of selected properties of materials and, more particularly, to the impedance measurement of such properties.
When dealing with materials it is often necessary to measure such properties as moisture content, composition, density and the like. Moisture content, for example, influences both the physical and chemical behavior of materials. In a physical sense, moisture content contributes to the overall weight of materials and is therefore an important factor in determining their invoice value for shipping purposes. In a chemical sense, moisture content is a factor in process control.
One technique for measuring a selected property of a material makes use of the fact that an electrical constant of the material, such as its dielectric constant, can be indicative of a selected property under test. For example, as the moisture content of a material increases, with other properties held constant, there is a corresponding increase in dielectric constant. In using this technique, it is customary to employ the material as the dielectric of a capacitive test cell. The cell may be calibrated so that an indicated change in capacitance is a measure of moisture content. Typically, an oscillator is employed with the test cell and measurements are made with different spacings between test cell electrodes.
The frequency shift resulting from the changed electrode spacing is indicative of the material's moisture content. In such systems, oscillator drift can create measurement errors. Also, such systems are generally ineffective for lossy materials in which a frequency signal deteriorates. Particular difficulties are encountered with materials having high moisture content of, for example, greater than 20%.
The object of this invention, therefore, is to provide an improved system for measuring predetermined properties of materials.